The present invention relates, in general, to pulse width modulation (xe2x80x9cPWMxe2x80x9d) methods and systems.
By way of definition and background, a PWM (pulse width modulated) signal is one in which the information is contained in the width of each pulse, typically of a repeating string of pulses. It may be considered a form of analog signal in that the information is contained in the time duration of a pulse, which is varied continuously, or in such small steps as to be effectively continuous. That is to be contrasted with a digital signal in which information is contained in discrete steps (such as two steps for binary) and in which values are assigned to the various discrete combinational possibilities.
Many circuits use PWM techniques for a variety of purposes. In one example, PWM is used to control most types of DC to AC converters , called inverters. PWM control may be used to control inverters in any of a variety of inverter applications, with examples including power supply applications such as uninterruptible powers supplies and the like, alternative energy applications such as fuel cells, motor drives, audio amplifiers, and the like.
There are many known methods for creating PWM signals. Conventional PWM sequences, for instance, may be generated by comparing a triangle or ramp carrier with a modulating function. Typically, signals are created that contain both high-frequency switching energy and low frequency waveform energy. Generally, the low frequency content limits the overall PWM applications. Inverters for motor drives, for example, can be complicated systems that must reconstruct gate drive waveforms and deliver power without transformer coupling. The desire to avoid transformers has hampered development of a xe2x80x9cgeneral purpose inverter.xe2x80x9d
A high frequency AC link inverter, in which a high-frequency transformer is inserted for power delivery, was proposed in xe2x80x9cHigh-frequency link power conversionxe2x80x9d, Espelage et al., IEEE Trans. Industry Applications, Vol. IA-13, pp. 387-394 (1977), herein incorporated by reference. Although this technology allows for power to be converted through a transformer, it has not been widely implemented due to its complexity. Indeed, most known high frequency link implementations require a generally complex, multistage power conversion design: an initial open loop inverter, the transformer, a rectifier, and then a final PWM inverter stage.
There are also multi-level inverters, which use several different dc input voltages to produce a waveform that tracks a modulating signal more closely than conventional two-level and three-level inverters. They are common in high-voltage applications, since the switches in them act in series. It is generally known that the control signals for multi-level inverters can be generated through a multiple-carrier PWM process. In such systems, the triangle carrier is shifted up or down to form a set of carriers corresponding to each output level. This approach, however, does not support high-frequency links or provide a way to simplify the inverter system itself. It intended solely to facilitate the use of many switches in a series configuration. Further descriptions of such systems may be found, by way of example, in, xe2x80x9cA new multilevel PWM method: a theoretical analysis,xe2x80x9d Carrara et al., IEEE Trans. Power Electronics, vol. 7, pp. 497-505 (1992).
Another method has been proposed to eliminate some of the multiple stages. xe2x80x9cHigh-frequency link DC/AC converter with suppressed voltage clamp circuitsxe2x80x94naturally commutated phase angle control with self turn-off devices,xe2x80x9d by Matsui et al., IEEE Trans. Ind. Applications, vol. 32, pp. 293-300, March/April (1996), herein incorporated by reference, discloses a naturally commutated cycloconverter-based inverter. No output or internal PWM signal is disclosed, however. Further, the disclosed method does not extend well to typical PWM applications that require precise PWM waveform and control.
Unresolved needs in the art therefore exist.
Generally, the present invention is directed to PWM systems and methods. A method of the invention has the steps of segmenting a base carrier waveform into a plurality of carrier waveforms, and providing at least one modulating signal. The plurality of carrier waveforms are compared to the at least one modulating signal to produce a plurality of comparator outputs, which are then mathematically combined to produce a pulse width modulated control signal. In a preferred embodiment of the invention, this pulse width modulated control signal is then convolved with a clock signal to produce a final pulse width modulated output.
An exemplary system of the invention for generating a gate drive sequence includes a segmenter operative to segment a base carrier waveform into a plurality of carrier waveforms, and a plurality of comparators linked to the segmenter and also linked to at least one modulating signal. The plurality of comparators are operative to compare the plurality of carrier waveforms to the at least one modulating signal. A mathematical operator is further provided that is linked to each of the comparators and operative to output the gate drive sequence defined through mathematical combination of the plurality of comparator outputs. In a preferred system for providing pulse width modulated output, a switching circuit is additionally provided. The gate control sequence and a square wave Signal are input to the switching circuit which uses the signals to produce a pulse width modulated output.
Methods and systems of the invention solve many otherwise unresolved problems of the prior art and provide valuable advantages. For example, through practice of multiple-carrier PWM methods and systems of the invention, relatively low cost and low complexity inverter designs can be achieved that support transformer isolation and offer other advantages. It is also possible to combine more than one modulating signal into a single output that entails independent PWM of each modulating signal. These and other advantages of the invention will be better appreciated through consideration of the invention embodiments described in detail below.